The present invention relates to a press in which the press frame is C-shaped and the slide is driven by a toggle mechanism, and which press is particularly suitable for cold forging.
Recently, processing by cold forging has been increasing because it is superior in productivity and stock utilization to cut and chip processing. Further, progress in methods of cold forging as well as quality and durability of dies has been remarkable, and there is an increasing tendency to utilize cold forging for processing materials which have heretofore been processed by cut and chip processing only. It would therefore be desirable to have improved forging presses.
Because a C-frame press is open at the front, it has an advantage over a straight-side press for convenience of operation. However, it has a shortcoming that it is liable to be deformed by the operational load. This deformation can be divided into two parts: one is deformation due to the horizontal component of the load, and it upsets the alignment between the punch and the die which has been set before operation, and changes the pre-adjusted correct clearance between them. This type of deformation thus has bad effects on both the operational precision and the durability of the dies. The other type of deformation is that due to the vertical component, and it acts to increase the distance between the punch and die, the so-called "open mouth" phenomenon. This deformation caused the punch to push very abruptly into the die due to the resilience or restoring force of the frame and other parts as soon as the operational pressure is no longer exerted on the punch during a blanking operation, i.e. as soon as the actual blanking operation is over although the punch is still in the material. This phenomenon is called "break-through", and it has a considerable effect on the durability of the dies.
As one measure to prevent the "open-mouth"phenomenon, it has heretofore been proposed to use tie rods across the opening of the frame. By this measure the opening of the frame is opposed by the tie rods secured by screws to the crown and bed parts of the press so that the frame is pre-deflected in a direction opposite to that of widening of the opening. However, because the frame is deflected in the direction to effect a "close mouth" phenomenon, excessive tightening causes considerable adverse deformation in this direction. Thus tie rods are not sufficient as a means to prevent the "open-mouth" phenomenon.
A toggle joint press travels speedily during the preliminary strokes, i.e. before the upper die mounted on the slide reaches the work piece, and its speed decreases during the actual working stroke so that it carries out the pressure-forming action at a slow speed. Thus, in a toggle joint press, as in a crank press, rotation of the motor is suitably decelerated, at the output thereof is changed into reciprocating movements through a crank and a connection, and a toggle mechanism causes the slide to carry out rectilinear compression forming.
Because in this type of press a large pressing power is obtained in the vicinity of the bottom dead center of the crank, it is suitable for forging.